Semiconductor memory devices, including flash memory, typically utilize memory cells to store data as an electrical value, such as an electrical charge or voltage. A flash memory cell, for example, includes a single transistor with a floating gate that is used to store a charge representative of a data value. Flash memory is a non-volatile data storage device that can be electrically erased and reprogrammed. Non-volatile memory retains stored information even when not powered, as opposed to volatile memory, which requires power to maintain the stored information. Increases in storage density have been facilitated in various ways, including increasing the density of memory cells on a chip enabled by manufacturing developments, and transitioning from single-level flash memory cells to multi-level flash memory cells, so that two or more bits can be stored by each flash memory cell.
Garbage collection is a process of memory management that reclaims portions of memory that no longer contain valid data. Using flash memory as an example, data is written to flash memory in units called pages, which are made up of multiple memory cells. However, flash memory is erased in larger units called blocks, which are made up of multiple pages. If some pages of a first block contain invalid data, those pages cannot be overwritten until the whole block containing those pages is erased. The process of garbage collection reads and re-writes the pages with valid data from the first block into a second block and then erases the first block. After garbage collection, the second block contains pages with valid data and free pages that are available for new data to be written. Since flash memory can only be programmed and erased a limited number of times, it is important to utilize a garbage collection scheme that maximizes or improves the life of a flash-based storage system.